Hyperbranched polymers are envisioned to possess interesting properties such as low viscosity and high thermal stability due to their substantially globular molecular architectures and to serve as functional materials. Realization of the full potential of hyperbranched polymers calls for the exploration of new, versatile methods for the preparation thereof. A variety of hyperbranched polymers have been prepared by condensation polymerization of so-called ABn-type (n≧2) monomers wherein A and B functions condense together to form branched polymers, for instance, hyperbranched polyester and polyamide polymers prepared by Frechet. J. M. J. (U.S. Pat. No. 5,514,764). In this preparation, however, competitive reactions often lead to the formation of linear chains, producing imperfect polymers with a low degree of branching and low molecular weight.
Cyclotrimerization of alkynes is a reaction where three alkynes cyclize into a benzene ring. This kind of [2+2+2] cycloaddition was first reported by Berthelot, M. in 1866 (Ann. Chim. Phys., 1866, 9, 446) and has now been developed into a widely used methodology in organic synthesis. Normally, the cyclotrimerization reaction of triple bonds requires the presence of transition metal catalysts (Chem. Comm., 1991, 19, 1392), which often results in the formation of mixtures of 1,2,4- and 1,3,5-substituted benzene regioisomers. Moreover, many transition metal catalyst complexes are functional group intolerant. In 1980, Balasubramanian, K reported for the first time the cyclotrimerization of benzoylacetylenes (Synthesis, 1980, 29). The reaction is known to be strictly regioselective and to yield only 1,3,5-triaroylbenzene (Org. Chem., 2002, 67, 4547). Another advantage of this reaction is that instead of a transition metal catalyst, the reaction proceeds refluxing in dimethylformamide (DMF) or in the presence of additional secondary amines as catalysts (Tetra. Lett., 2000, 41, 6545 and J. Org. Chem. 2002, 67, 4547). The ethynylketones are prepared from substituted benzaldehydes through reaction with ethynylmagnesium bromide and followed by oxidation with MnO2 or CrO3. However, the synthesis of the benzoylacetylenes is a multi-step procedure involving organometallic complexes such as ethynylmagnesium bromide and toxic heavy metal oxides such as MnO2 or CrO3. Additionally, strict reaction conditions such as exclusion of air and moisture need to be applied.
Cycloaddition of diacetylenes (or diynes) has been used as an elementary reaction for the construction of hyperbranched polymers with high molecular weight and degree of branching because the polycyclotrimerization mechanism is intolerant toward the formation of linear repeat units inside the hyperbranched core. For instance, Olsen, R. E. prepared a novel polyphenylene resin by copolycyclotrimerization of vinylethynylbenzene, diethynylbenzene, and phenylacetylene (U.S. Pat. Nos. 5,686,027 and 5,594,058) initiated by Ziegler catalysts. The obtained polymer is used in the construction of high-performance carbon-carbon composites. This reaction system is, however, complex. Ben Zhong Tang's group has successfully synthesized soluble hyperbranched poly(alkenephenylene)s and poly(arylene)s by polycyclotrimerization of diynes with tantalum and cobalt complexes as the catalysts (Macromolecules, 2007, 40, 1914, Macromolecules, 2004, 37, 5196 and C. R. Chimie, 2003, 6, 833). However, as the methods for polymerization mentioned above involved a transition metal catalyst and yielded a regioirregular polymer, some scientists have tried to apply regioselective methods for the synthesis of hyperbranched polymers. Tiwari, R. K. reported a branched polymer containing triaroylbenzene moieties (Indian Journal of Chemistry, 1996, 35B, 1263) and a process for making the same. However, the resultant polymer is not soluble in most organic solvents, which makes the characterization and the application of the polymer difficult.
Tang, has successfully synthesized soluble triaroylbenzene-based branched (co)polymers (U.S. patent application Ser. No. 11/417,290). The resultant polymers are soluble in common organic solvents, have high thermal stability and photosensitivity and function as photoresist materials. The monomer building blocks, however, require the presence of an aromatic moiety next to the carbonylacetylene group (aroylacetylene or diaroylacetylene), which are synthetically only accessible through multi-step procedures involving moisture sensitive organometallic complexes and toxic heavy metal compounds.
Accordingly, there has been a need to develop an easy and fast one-step procedure with a mild condition for the preparation of hyperbranched polymers, as well as their monomer building blocks.
In the present invention, the cyclotrimerization of propiolates in the absence of metal has been discovered and for the first time the cycloaddition of bis(acylacetylene)s has been used to prepare processible hyperbranched (co)poly(acylarylene)s.